I have observed the same effect with IEC power filters that Andy describes, and have heard this tale of woe from a lot of fellow DIYers as well. I have not investigated this issue in detail, so much to do - so little time.. LOL My only surmise is that the linearity of the inductors and caps in those filters are suspect at audio frequencies and perhaps in addition they raise the source impedance of the ac supply. I have built filters using hand wound common mode and diff mode chokes and good film caps and heard some significant benefit in a few cases.

As I mentioned earlier, it's there for reliability- I've had too many pots go funny in the wiper, and the resistor there keeps the pops and bangs away in event of that failure. But indeed, it's optional. At about $0.02, it's a cheap measure.

Dave: I saw them live about 10 years ago. Everyone in the audience was my age, +/- two years. As are you, apparently.

Hi RDF,
Sorry for the tardy reply been distracted by work. The difference I hear with no rfi filter compared to with rfi filter is the rfi filter seems to make everthing sound closed down. The highs aren't as clean and open sounding and the mids aren't as open. The bass with rfi filter doesn't seem to be as articulate.
Micro Henry size chokes in the high voltage supply really aren't large enough. You would need something very much larger to do a good job of filtering.

Thanks Andy. My experience with the uH coils somewhat echoes your experience, though I hear it as a suprisingly cleaner top end. To my ears the result is incredibly 'fast' for a tube amp over the speakers at hand (Jordan JX92s and 12" Tannoy Gold), albeit based on a sample of one so far.

Quote:

Originally posted by Andy Bartha You would need something very much larger to do a good job of filtering.

Depends on the target. The coils are sized to do nothing at frequencies lower than the self-resonance of the ASC oil power supply caps, but at frequencies above ~200 kHz in combination with RC bypass shunts simulations suggest a very large improvement in suppression of RF from the mains. That's a topic for a new thread though.
Many thanks again for your input.

I havenít diagrammed input switching since that will vary a bit from installation to installation. Use a good quality switch; I used a surplus Cinema Engineering 5 position two deck rotary switch with shorting between positions for optimal isolation. The input grounds are all tied together and kept completely separate from the preamp ground

What would be the disadvantage of using a 4-pole switch and source switching both signal and ground?
My intuition says that if there are ground issues between sources, its best to not tie them together.
By the way, every time my intuition is wrong, I learn something.

For the last 15 years I've used a passive control/volume unit based on a 4 pole Greyhill. It switches grounds and only the selected source and pre chassis are connected to the amp while in use. It's a make before break with audio sources wired every second position. In between sources the hot side has 100 ohm resistors wired to the ground terminals, themselves always referenced to the RCA outs. The net effect is sources are momentraily loaded with 100 ohms during switching, in normal use the resistors short any switch capacitance to ground to very high frequency.

I have never heard even a hint of inter-source crosstalk in any system it's been used. The amp I'm using now has a great deal of gain and would expose any issues. The couple of times I measured it with an Audio Precision test set the interchannel crosstalk exceeded 120 dB, at a level where shifting internal wires had an effect.